The present invention relates to a bidirectional wavelength division multiplex transmission apparatus using a wavelength division multiplex optical amplifier.
FIG. 7 shows the first example of the conventional bidirectional transmission apparatus using the bidirectional wavelength division multiplex optical amplifier. As shown in FIG. 7, this apparatus uses WDM (Wavelength Division Multiplex) couplers 21 to 23 as optical branching/inserting means for signal light beams to be transmitted bidirectionally. Reference numeral 2 denotes an optical multiplexer; and 3, a wavelength division multiplex transmission optical amplifier.
FIG. 8 shows the second example of the apparatus using the bidirectional wavelength division multiplex optical amplifier. As shown in FIG. 8, this apparatus uses optical circulators 31 and 32 to make only the optical transmission lines bidirectional. The apparatus amplifies branched wavelength division multiplex signals by using wavelength division multiplex transmission amplifiers 41 and 42 separately used for reverse and forward links.
In the apparatus as the first example using the WDM couplers, the respective wavelengths must be spaced apart from each other by at least about 10 nm, as indicated by the wavelength transmission range characteristics in FIG. 9. For this reason, the number of wavelengths which can be multiplexed is limited, and optical transmission lines for transmitting optical signals cannot be effectively used.
The apparatus as the second example, which makes only the optical transmission lines bidirectional, requires two optical amplifiers. This increases the cost of the apparatus and degrades maintainability.